Flat bits are commonly used for drilling of holes with diameter 10 to 40 mm (3/8 to 1 1/2 inch) in wood with electric hand drills. Since they are not self-feeding it is easy to control the penetration into the workpiece, and they are thus used above all for shallow holes and for through holes in relatively thin workpieces.
Simple flat bits, described in the U.S. Pat. Nos. 2,794,468 and 4,066,379 consist of a shaft and a flat part made as a plate with substantially even thickness, a triangular pilot point, two substantially radial main cutting edges and two substantially parallel non-cutting longitudinal edges. The shaft is attached to the flat part by welding, by brazing or by a threaded connection. It is in general required that flat bits should be possible to resharpen by filing.
The purpose of the pilot point is to commence the drilling of the hole at a well defined center, to stabilize the drill bit when the radial edges start to cut in order to get a clean non-splintered perimeter at the entry side, and when a through hole is to be drilled to make a pilot hole with small diameter in order to locate and guide the drill when the last part of the hole is drilled from the reverse side of the workpiece.
The purpose of the main radial edges is to cut and transport chips with a low and even force, and to form the major part of the bottom surface of a dead end hole.
These different purposes set partly conflicting requirements for the design of the flat bit, and it has been difficult to satisfy all to an acceptable extent, in spite of many improvements on the original design. A pilot point with even thickness offers acceptable stability but has a large negative rake angle, leading to high cutting force, unsatisfactory chip transport and overheating when drilling in resinous or hard wood. It will also deviate to the side if the hardness of the workpiece is not homogeneous, as in drilling parallel to the fibers or near a knot. The same problems but less serious are found with pyramid-shaped points. A pilot point which has grooves along the edge according to U.S. Pat. No. 2,782,824 has positive rake angle, lower cutting force and better defined position, but will not stabilize the drill well enough when the force on the the main cutting edges varies. All triangular pilot points will also cause a parallel error and lack of accuracy when a through hole is drilled at some other angle than normal to the surface of the workpiece.
A drill bit with a pilot point with partly parallel sides is described in U.S. Pat. No. 2,543,206 and provides good stability and good guiding in through holes, but other features of this drill bit restrict it to drilling with high rpm, slow penetration and limited diameter. Pilot points where the width decreases near the main radial edges are known from U.S. Pat. No. 3,920,350 and lead to somewhat less cutting force but less stability.
The main radial cutting edges are normally located at an angle of 70 to 90 degrees to the drill axis, 90 degrees giving the lowest cutting force when drilling at right angle to the fibres but a greater risk of uneven or splintered perimeter except when drilling with very low feed rate. A smaller angle produces a neater perimeter, less vibrations when entering the workpiece and has less requirements for stabilizing, but needs more cutting force and wears more rapidly. If grooves are made parallel to the main radial edges the rake angle will be positive and the cutting force lower, but the perimeter less neat. It is known from the patent U.S. Pat. No. 4,682,917 to make spurs protruding from the outer ends of the main radial edges in order to sever the fibers before they are touched by the main radial edges. This allows main radial edges at 90 degrees to the drill axis and positive rake angle, producing neat holes with low cutting force, but this type of drill cannot easily be resharpened.